Arrayed waveguide grating

ABSTRACT

An arrayed waveguide grating (AWG) device is described in which there are a plurality of output waveguides ( 10 ) coupled at one end to the output side of an second optical interaction region ( 4 ) of the AWG, and wherein the output waveguides are substantially identically curved in at least a portion of the fan-out region of the AWG. This improves the channel (frequency) spacing accuracy of the AWG. In one embodiment input waveguides ( 2 ) of the AWG are also substantially identically curved in at least a portion of the fan-in region of the AWG, these being inversely curved to the substantially identically curved portions of the output waveguides, with respect to the direction of travel of light along the waveguides. This has been found to reduce asymmetry in the channel output response.

[0001] The present invention relates to dispersive optical devices. Morespecifically, but not exclusively, the invention relates to an arrayedwaveguide grating device.

[0002] In order to meet the ever-increasing demand for transmissionbandwidth in communication networks, operators are investing heavily inthe development of techniques for Dense Wavelength Division Multiplexing(DWDM). DWDM employs many closely spaced carrier wavelengths,multiplexed together onto a single waveguide such as an optical fibre.The carrier wavelengths are spaced apart by as little as 50 GHz in aspacing arrangement designed in the style of an ITU (InternationalTelecommunications Union) channel “grid”. Each carrier wavelength may bemodulated to provide a respective data transmission channel. By usingmany channels, the data rate of each channel can be kept down to amanageable level.

[0003] Clearly, to utilize this available bandwidth it is necessary tobe able to separate, or demultiplex, each channel at a receiver. Newoptical components for doing this have been designed for this purpose,one of these being the Arrayed Waveguide Grating (AWG). An arrayedwaveguide grating is a planar structure comprising a number of arrayedwaveguides which together act like a diffraction grating in aspectrometer. AWGs can be used as multiplexers and as demultiplexers,and a single AWG design can commonly be used both as a multiplexer anddemultiplexer. A typical AWG mux/demux 1 is illustrated in FIGS. 1(a)and (b) and comprises a substrate or “die” 1 having provided thereon oneor more input waveguides 2 for a multiplexed input signal, two slabcouplers 3,4 connected to either end of an array 5 of transmissionwaveguides 8, only some of which are shown, and a plurality of outputwaveguides 10, which are commonly single mode or substantially singlemode waveguides, for outputting respective wavelength channel outputsfrom the second (output) slab coupler 4 to the edge 12 of the die 1. Thearray 5 of transmission waveguides exhibits dispersive imagingproperties like that of a diffraction grating, so that input WDM signalsare dispersed and focused to respective ones of the output waveguides.Often, the input and/or output waveguides 2,10 are each tapered (usuallyadiabatically tapered) at a respective first end 7,11 thereof, wherethey are coupled to the first or second slab coupler respectively, thetaper being such that the width of the waveguide increases towards theslab coupler 4,5. The output waveguides 10 are arranged to “fan-out”from their first ends 11, away from each other and from the coupler(i.e. the lateral spacing between the waveguides is increased), so as toachieve a desired physical spacing between the output waveguides, at anoutput edge of the device.

[0004] One known problem with such AWG devices is that the channeloutputs from the output waveguides tends to deviate from the idealdevice response in which the central frequencies of the channels areequally spaced apart, as illustrated in FIG. 2(a). Instead, the outputfrom each channel is often not centred on the respective desiredfrequency f₁, f₂, f₃, . . . but is instead centred on a slightlydifferent frequency. We believe these inaccuracies in channel spacingresult, at least in part, from the shape and arrangement of the outputwaveguides, further influenced by manufacturing process aberrationsintroduced during manufacture of the device. In particular, we believethe radius of curvature of each of the output waveguides, particularlyin the fan-out region where the output waveguides fan-out from thesecond slab coupler, is a significant contributing factor in the channelspacing inaccuracies experienced in the manufactured devices,particularly where tapers are used on said first ends 11 of the outputwaveguides.

[0005] It is an object of the present invention to avoid or minimize oneor more of the foregoing disadvantages.

[0006] According to a first aspect of the invention there is provided anarrayed waveguide grating device comprising: first and second opticalinteraction regions between which an input optical signal propagatesfrom a first position on a first side of the first optical interactionregion to a second position on a second side of the second opticalinteraction region, a correspondence between said first and secondpositions depending upon a wavelength of the optical signal; a pluralityof array waveguides coupled between a second side of the first opticalinteraction region and a first side of the second optical interactionregion; and a plurality of output waveguides coupled at one end to thesecond side of the second optical interaction region; wherein the outputwaveguides are arranged to fan-out from the second optical interactionregion, in a fan-out region of the device which comprises an initialportion of the length of each output waveguide, and the waveguides inthe fan-out region are substantially identically curved in at least aportion of the fan-out region.

[0007] The device according to the invention has the advantage ofimproved channel spacing accuracy i.e. avoiding, or at least minimizing,deviations in channel spacing from the desired, ideal channel spacingwhich the device is designed to achieve.

[0008] The waveguides in the fan-out region preferably all havesubstantially the same radius of curvature in at least a portion of thefan-out region. Alternatively, the waveguides may each have acontinuously varying radius of curvature, in the fan-out region, inwhich case the radius of curvature of each waveguide variessubstantially identically to that of the other waveguides, alongcorresponding portions of the lengths thereof, in at least a portion ofthe fan-out region.

[0009] Preferably, the waveguides are identically, or near identically,curved in at least a portion of the fan-out region, but alternativelythere may be small variations in the radius of curvature between any twoor more of the output waveguides, but preferably no two outputwaveguides differ in radius of curvature by more than 5 mm,advantageously no more than 1 mm, and most preferably no more than 0.5mm.

[0010] Preferably, the output waveguides are single mode, orsubstantially single mode, waveguides which are preferably tapered inwidth at the ends thereof which are coupled to the second opticalinteraction region, so as to increase in width towards the secondoptical interaction region.

[0011] Preferably, the portions of the waveguides having substantiallyequal radius of curvature, or substantially identically varying radiusof curvature, are adjacent the respective tapered ends thereof.Preferably, the radius of curvature of these portions is keptsufficiently low to substantially filter out any psuedo higher ordermodes which may arise where the output waveguides are coupled to thesecond optical interaction region, while still being sufficiently highto avoid unacceptably high radiation losses in the output waveguides.

[0012] The waveguides preferably each comprise a core region havingcladding material at least on either side of the core region. In ourpreferred embodiment, the difference between the refractive indices ofthe core region and cladding material is 0.01 and the core size is 6×6μm in cross-section, and the radius of curvature of the waveguides inthe portion of the fan-out region in which the waveguides aresubstantially identically curved is preferably in the range of 16 mm orless, preferably between 5.5 and 10 mm, most preferably substantially 8mm.

[0013] The device may conveniently be a planar waveguide-type devicewhich is generally rectangular in shape, having an input edge, an outputedge, and two side edges. A second end of each output waveguide mayconveniently lie along an output edge of the device. Advantageously, theoutput waveguides may be substantially identically curved throughout asubstantial portion of the fan-out region, preferably in the range of3-20% of the length of the fan-out region, most preferably between 5 and10%, as measured along a said side edge of the device which isperpendicular to the output edge of the device.

[0014] The device may further include at least one input waveguidecoupled to a first side of the first optical interaction region. The oreach input waveguide is preferably also a single mode, or substantiallysingle mode, waveguide and may also be tapered in width at the endthereof which is coupled to the first optical interaction region, so asto increase in width towards the first optical interaction region. Wherethere are a plurality of input waveguides, the input waveguides arepreferably arranged to fan-in towards the first optical interactionregion, in a fan-in region which comprises a portion of the length ofeach input waveguide, and all the input waveguides may also besubstantially identically curved in at least a portion of the fan-inregion.

[0015] Preferably, the substantially identically curved portions of theinput waveguides are substantially inversely curved to the saidsubstantially identically curved portions of the output waveguides, withrespect to the direction of travel of light along the waveguides. Wherethe input waveguides are tapered in width at the ends thereof where theyare coupled to the first optical interaction region and the outputwaveguides are tapered in width at the ends thereof where they arecoupled to the second optical interaction region, the substantiallyidentically curved portions of the input waveguides are preferablyadjacent the respective tapered ends thereof, the substantiallyidentically curved portions of the output waveguides are preferablyadjacent the respective tapered ends thereof, and said substantiallyidentically curved portions of the input waveguides preferably have aradius of curvature of equal magnitude to the radius of curvature ofsaid substantially identically curved portions of the output waveguides.

[0016] According to another aspect of the invention there is provided anarrayed waveguide grating comprising:

[0017] a substrate having first and second slab couplers;

[0018] a plurality of array waveguides optically coupled between thefirst and second slab couplers and having respective predeterminedoptical path length differences therebetween;

[0019] at least one input waveguide optically coupled at a first endthereof to an input side of the first slab coupler;

[0020] a plurality of output waveguides optically coupled at first endsthereof to an output side of the second slab coupler;

[0021] the output waveguides are arranged to fan-out from the secondslab coupler, in a fan-out region of the device which comprises aninitial portion of the length of each output waveguide; wherein

[0022] the output waveguides in the fan-out region are substantiallyidentically curved in at least a portion of the fan-out region,preferably proximal to the first ends of the output waveguides, and acorresponding portion of the or each said input waveguide issubstantially inversely curved to said substantially identically curvedportions of the output waveguides, with respect to the direction oftravel of light along the waveguides.

[0023] Preferably, the substantially identically curved portions of theinput waveguide(s) have a radius of curvature of equal magnitude to theradius of curvature of the corresponding substantially identicallycurved portions of the output waveguides.

[0024] The advantage of the inversely curved waveguide portions is thatthis compensates, at least to some extent, for the asymmetric effectswhich the bends in the waveguides, principally in the regions adjacentthe slab couplers, have on the shape of the channel output (spectral)response of the AWG, so that a substantially symmetric channel outputresponse is obtained.

[0025] Preferred embodiments of the invention will now be described byway of example only and with reference to the accompanying drawings inwhich:

[0026]FIG. 1(a) is a schematic plan view of a known arrayed waveguidegrating device;

[0027]FIG. 1(b) is a magnified schematic view of the ringed portion A ofthe device of FIG. 1(a);

[0028]FIG. 2(a) illustrates an ideal channel spacing, in terms offrequency, of the output channels of the device of FIG. 1;

[0029]FIG. 2(b) illustrates a typical channel spacing, in terms offrequency, of the output channels of the device of FIG. 1;

[0030]FIG. 3(a) shows an arrayed waveguide grating according to theinvention;

[0031]FIG. 3(b) is a magnified schematic view of the ringed portion A′of the device of FIG. 3(a);

[0032] FIGS. 4(a) and (b) compare schematically the AWG of FIGS. 3(a)and (b) with a modified version of the AWG according to anotherembodiment of the invention;

[0033]FIG. 5 is a graph of Transmission vs. Relative frequencyillustrating the output response of one channel of an AWG according toFIGS. 3(a),(b) as compared with the output response of the same channelin an AWG incorporating the modification illustrated in FIG. 4(b);

[0034] FIGS. 6(a) and (b) illustrate in greater detail identicallycurved portions of input and output waveguides in the AWG of FIGS. 3(a)and (b);

[0035] FIGS. 7(a) and (b) illustrate in greater detail the same portionsof the input and output waveguides, incorporating the modification ofFIG. 4(b).

[0036] FIGS. 3(a) and (b) illustrate respectively an arrayed waveguidegrating (AWG) device, and the arrangement of output waveguides 10′ in anAWG, according to one embodiment of the invention. Like components tothose of FIG. 1(a) are referenced by like reference numerals. The AWGdevice is formed on a substrate or “die” 1 and comprises at least oneinput waveguide 2, a first optical interaction region in the form of aslab coupler 3, an array of waveguides 8 (only some shown) of differentoptical path lengths and arranged between the first slab coupler and asecond slab coupler 4 (providing a second optical interaction region),and a plurality of output waveguides 10′ (only six shown). The outputwaveguides are single mode waveguides, or alternatively substantiallysingle mode waveguides (in the sense that overall they effectivelyoperate as if they were single mode). In generally known manner there isa constant predetermined optical path length difference between adjacentwaveguides in the array, which determines the position of the wavelengthoutput channels on an output face 5 of the second slab coupler 4.Typically, the physical length of the waveguides increases incrementallyby the same amount, ΔL, from one waveguide to the next, where

ΔL=mλ _(c) /n _(c)

[0037] where λ_(c) is the central wavelength of the grating, n_(c) isthe effective refractive index of the array waveguides, and m is aninteger number. The construction and operation of such AWGs is wellknown in the art and is described, for example, in “PHASAR-basedWDM-devices: principles, design and applications”, M K Smit and C. vanDam, J. Selected Topics in Quantum Electronics 2, 1996, pp236-250, andin “An N×N optical multiplexer using a planar arrangement of two starcouplers”, C. Dragone, Photonics Technology Letters, 9, 1991, vol 3,pp812-815.

[0038] The output waveguides 10′ are each tapered adiabatically at afirst end 11 thereof, where they are coupled to the output face 5 of thesecond slab coupler 4, in like manner to the device of FIGS. 1(a) and(b). The output waveguides 10′ also “fan-out” from their first ends 11′towards the output edge 12 of the device, so as to achieve a desiredphysical separation of the output channels at the output edge 12 of thedevice. The area of the device in which the lateral separation of thewaveguides increases, from their spacing at the output face 5 of thesecond slab coupler 4 to the final desired lateral spacing, ishereinafter referred to as the fan-out region. In the fan-out region thewaveguides are curved. Output end portions 10 b′ of the outputwaveguides 10′, adjacent the output edge 12, are substantiallyperpendicular to the output edge 12, in plan view of the AWG device.References made hereinafter to the length F of the fan-out region referto the length of the fan-out region as measured along a bottom edge 14of the device, which edge is perpendicular to the output edge 12, asshown in FIG. 3(a).

[0039] The device is provided as a planar silica-on-silicon chipproduced by, for example, Flame Hydrolysis Deposition (FHD) of ChemicalVapour Deposition (CVD). Each of the waveguides 2,8,10′ is of typicaloptical waveguide construction comprising a core region with a claddingmaterial at least on either side of, and in our preferred embodimentsalso covering, the core region. In generally known manner, the coreregion is formed on a substrate of silicon, silica (SiO₂) or the like,which may (if not made of silica) have a silica buffer layer depositedthereon before the core and cladding regions are deposited.

[0040]FIG. 3(b) illustrates the arrangement of output waveguides 10′ inthe fan-out region. For clarity, only four of the output waveguides 10′are shown in FIG. 3(b). In practice there would be many more, forexample 40 output waveguides for a 40-channel mux/demux. In a portion Pof the fan-out region, this portion being adjacent the tapered ends 11of the output waveguides, all the output waveguides have the same radiusof curvature. In the illustrated embodiment, this “equal curvature”portion P comprises a substantial portion of the fan-out region,specifically in the range of 5 to 10% of the length F of the fan-outregion. In the embodiment of FIG. 3 (which is not shown to scale), thelength of the bottom edge 14 of the AWG device is 50 mm, and the “equalcurvature” portion of the output waveguides spans approximately 1 mm ofthis length.

[0041] After these “equal curvature” sections of the waveguides, asillustrated in FIG. 3(b), the waveguides diverge so that some have anopposite radius of curvature to others. In fact, approximately half theoutput waveguides curve outwardly away from the other half of thewaveguides, and vice versa. This particular arrangement is advantageousin that it allows greater compactness of the overall device to beachieved.

[0042] Beam Propagation Method (BPM) simulations have shown that theradius of curvature of the output waveguides, at the input ends 11thereof, can affect the output channel spacing of the AWG device, inparticular output waveguides with different radii of curvature can leadto inaccuracies in channel spacing. This is highly undesirable, asaccurate channel spacing, in particular in terms of channel frequency,is a desired feature of any commercial AWG device. Our simulations alsoshow that the detrimental effects due to different radii of curvatureare greater the wider the tapers used on the input ends 11 of the outputwaveguides.

[0043] Additionally, it is known that output waveguides which are notexactly coupled to the second slab coupler 4 may excite higher orderpsuedo-modes in substantially single mode output waveguides whichdistort the optical field at the wide end of the taper. BPM simulationsalso show that this effect can be reduced by reducing the radius ofcurvature of the waveguides in the fan-out region. A narrower curve(i.e. smaller radius of curvature) can, at least to some extent, filterout such psuedo higher order modes, which a wider curve would not.

[0044] Therefore, the radius of curvature of the “equal curvature”portions of the output waveguides is chosen to be relatively small,while still within acceptable radiation losses for the outputwaveguides. In the described embodiment, we chose a radius of curvatureof 8 mm, for a waveguide core size of 6×6 μm, and where the difference,An, of the refractive indices of the waveguide core and cladding is0.01. We have found that for this core size, with this value of Δn, theradiation losses in the output guides are acceptable. For thisembodiment, if a radius of curvature of less than 8 mm is used, theradiation losses become higher, particularly for below 5.5 mm where wehave found these losses are unacceptably high. It will thus be generallyunderstood that for waveguide designs with different core size and/or Δnvalue to the described embodiment, the radius of curvature of the “equalcurvature” sections should ideally be chosen so as to be high enough toavoid unacceptable radiation losses, but also low enough that thewaveguides do function to filter out, or at least partially filter out,the psuedo higher order modes. This can be done empirically and/or viasimulation. A working value for the maximum tolerable additional powerloss attributable to radiation losses (i.e. over and above power lossdue to other factors) in an output waveguide, due to bending of thewaveguide, is generally accepted to be about 0.1 dB for a ninety degreebend in the waveguide.

[0045] In general, the longer the length of the “equal curvature”portion of the fan-out region (measured along the bottom edge 14), thelarger will need to be the overall size of the AWG device. As it isusually desirable to make the AWG device as small as possible, thedesigner must therefore balance the benefits of using a larger “equalcurvature” portion (potentially better filtering out of pseudo higherorder modes and/or improved channel spacing accuracy) against thecorresponding increase in overall size of the device.

[0046] As a further improvement, the input waveguides 2 may also bedesigned so that a portion of each input waveguide adjacent the firstslab coupler 3 is substantially identically curved to a correspondingportion of all the other input waveguides, the radius of curvature againbeing chosen to be as low as possible while still high enough to avoidundesirably high radiation losses. This can be beneficial to filter out,or at least partially filter out, any pseudo higher order modes whichmay be present in the input waveguides. In the embodiment of FIG. 3(a)the input waveguides are single-mode, or substantially single-mode,waveguides which fan-in towards the first slab coupler 3. The inputwaveguides are adiabatically tapered at the ends thereof which areconnected to the first slab coupler 3, and are all substantiallyidentically curved for at least a portion of the fan-in region.

[0047] It will be appreciated that further variations and modificationsto the above-described embodiment are possible without departing fromthe scope of the invention. For example, instead of the outputwaveguides having one, equal, radius of curvature along at least aportion of the length thereof in the fan-out region, the radius ofcurvature of the waveguides may vary continuously along the length ofeach waveguide, or along at least a portion thereof. (For example, thepath of each waveguide might be defined by a polynomial function.) Inthis case, according to the invention the radius of curvature of eachwaveguide would be varied identically to that of the other waveguides,along at least a portion of the length of the waveguides in the fan-outregion. i.e. the waveguides would all have the same curvature alongcorresponding portions thereof, in at least a portion of the fan-outregion.

[0048] Although the output waveguides are ideally identically curved inthe portion P of the fan-out region, as described with reference to theembodiment of FIGS. 3(a) and (b), small variations in the radius ofcurvature are tolerable, within limits, while still achieving at leastsome of the benefit of the invention. We believe that variations inradius of curvature of no more than 5 mm between any two of the outputwaveguides is tolerable, with variation of no more than 1 mm, ideally nomore than 0.5 mm, being most preferable.

[0049] In a further possibility instead of, or in addition to, tapers onthe first ends 11 of the output waveguides, there may be a multi-modeinterferometer (MMI) disposed between the second slab coupler 4 and theinput ends 11 of each of the output waveguides, the slab coupler andMMIs together forming a desired “optical interaction region” between theoutput waveguides 10′ and the array waveguides 8.

[0050] In another possible modified embodiment, there may be no tapers(or MMIs) on the input ends 11 of the output waveguides 10 (i.e.adjacent the second slab coupler 4). For example, in this embodiment theoutput waveguides may be multi-mode waveguides, for example two-modewaveguides transmitting both the fundamental and first order modes.While an embodiment with no tapers on the output waveguides may not beso susceptible to channel inaccuracies introduced due to unequalcurvature of the output waveguides in the fan-out region, we envisagethat the use of “equal curvature” waveguide sections according to theinvention will still provide some benefit.

[0051] In a yet further embodiment there could be more than onesingle-mode output waveguide allocated for each output channel. Forexample, a respective pair of single mode waveguides may be provided tooutput each channel.

[0052] One problem associated with the use of bends in the outputwaveguide sections is that the bends in the output waveguides adjacentthe fan-out region tend to introduce an asymmetry into the outputchannel response. This asymmetry is due to the fact that where theoutput waveguide bends, the mode field of the optical signal beingtransmitted therein is distorted as the mode is pushed outwardly awayfrom the centre of the arc of the bend (so that the mode is no longersymmetrical about the optical axis of the waveguide). Where the inputwaveguides also bend in the fan-in region in the manner shown in FIGS. 2and 3(a), this tends to further exacerbate the asymmetric nature of thechannel output response (also sometimes referred to as the transmissionspectrum).

[0053] FIGS. 6(a) and (b) show in more detail corresponding input andoutput waveguide portions of an AWG of the type shown in FIG. 3(a) inwhich the input waveguides are identically curved in a portion of thefan-in region adjacent the tapered input waveguide ends at the inputslab and the output waveguides are all identically curved (to oneanother) in a corresponding portion of the fan-out region adjacent thetapered output waveguide ends at the output slab. The input waveguidesand the output waveguides all bend towards the bottom edge of the die 1(in plan view of the die), with respect to the direction of travel oflight along the waveguides. The grey line plot in FIG. 5 illustrates theresulting asymmetric channel output response for this AWG. The frequencyvalues are given relative to the center channel frequency (which isrepresented as zero on the Rel. Frequency axis).

[0054] One consequence of the asymmetric output response is that itleads in practice to greater crosstalk between adjacent output channelsthan the AWG was designed for, which degrades the AWG performance ascompared with the predetermined ITU grid response for which the AWG isdesigned.

[0055] Therefore, in a modified embodiment of the invention, we proposethat the input waveguides 2 all have inverse curvature to theidentically curved portions of the output waveguides, with respect tothe direction of travel of light along the waveguides, for acorresponding portion of the lengths thereof in the region adjacent theinput slab coupler 3. FIG. 4(a) shows schematically an AWG as describedabove with reference to FIGS. 6(a) and (b). FIG. 4(b) showsschematically the improved AWG having the inverse bend on the inputwaveguides. For clarity, only one input waveguide is shown in each caseand the bend on the input waveguides has been heavily magnified (ascompared with the actual device).

[0056] For comparison with FIGS. 6(a) and (b), FIGS. 7(a) and (b) showthe corresponding input and output waveguide sections in the improvedembodiment in which the input waveguides are inversely curved (withrespect to direction of light travel) to the output waveguides, with thesame magnitude of radius of curvature to the output waveguides, incorresponding (same length) portions adjacent the first and second slabs3,4 respectively. Thus, in FIG. 7 the input waveguides bend towards thetop edge of the AWG die (in plan view thereof), while the outputwaveguides bend towards the bottom edge. In this design the fan-in andfan-out regions can be considered as being “rotation-symmetric”.

[0057] The input and output waveguides are preferably adiabaticallytapered at the ends thereof which are coupled to the first and secondslabs respectively, as in the first above-described embodiment. Theinverse bending of the input waveguides compensates for the asymmetriceffect on the output response due to waveguide bending, to produce amore symmetric output response as illustrated in the black line graph Bin FIG. 5. The adjacent crosstalk is therefore improved (adjacentcrosstalk is reduced).

[0058] The reasons why a more symmetric response is obtained will be nowbe described briefly. An inherent characteristic of an AWG is that inthe main image (or “focal”) field, at the output side 5 of the slabcoupler, the imaged mode field is substantially a mirror image of theinput mode field (it is distorted to at least some extent due to thenature of the phase array properties of the AWG). The transmissionspectrum (i.e. output spectral response) of each AWG channel is ineffect a convolution of the image of the input field which appears atthe output side 5 of the (output) slab coupler, with the (eigen-) modeof the respective output waveguide 10. Where one or both of the inputand output fields is asymmetric, the resulting transmission spectrumwill in general by asymmetrical. The degree of asymmetry is determinedby the shape of the input and output fields as well as their mutualorientation. Assuming perfect imaging of the AWG, the shape of thefields is mainly determined by the transmission properties of the fan-inand fan-out regions. The mutual orientation is determined by theorientation of the fan-in region with respect to the fan-out region. Itwill thus be appreciated that if an inverse bend is used on the inputwaveguide to the bend used on the output waveguides (at least forcorresponding initial portions of the fan-in and fan-out regions), withrespect to the direction of travel of light along the waveguides, theasymmetric mode field input by the bent input waveguide to the inputslab coupler 3 is (substantially) mirrored in the image plane at theoutput side of the second slab 4, thus substantially matching theasymmetric mode of the bent output waveguide. This results in aconvolution of two substantially identical asymmetric responses, whichresults in a symmetrical channel output response (i.e. transmissionspectrum) as illustrated in the black line graph in FIG. 5. (Bycomparison, in the original embodiment of FIGS. 6(a) and (b) it will beapparent that when the asymmetric mode in the bent input waveguide isgenerally mirrored by the AWG the resulting asymmetric mode field imagedat the output side 5 of the output slab coupler 4 will be generally (butnot exactly) oppositely skewed to the asymmetric mode of the similarlybent output waveguide, so that the resulting convolution is stillasymmetric.)

[0059] Although in the ideal embodiment the relevant (substantiallyidentically bent) portions of the input waveguides will have exactly thesame magnitude radius of curvature, but be inversely curved, to thecorresponding (substantially identically bent) portions of the outputwaveguides, it will be understood that some variation between the radiusof curvature of the corresponding bent input and output waveguideportions is tolerable without preventing a more symmetric channel outputresponse being achieved, although the full benefit of exactly matchingthe magnitude of the input and output bends will not be achieved. Ingeneral, a variation of at least a few millimeters is likely to betolerable, but preferably no more than this, most preferably no morethan 0.5 mm variation, is used.

[0060] It will be appreciated that the terms “input” and “output”, asused above to describe the waveguides, are used with reference to theuse of the AWG as a demultiplexer. However, the same AWG could equallybe used as a multiplexer in which case the input and output waveguidesfunction as output and input waveguides, respectively. Therefore theaccompanying claims shall be read as covering an AWG suitable for use asa demultiplexer, a multiplexer or a device which can function as both(i.e. a mux/demux device).

1. An arrayed waveguide grating (AWG) device comprising: first andsecond slab couplers between which an input optical signal propagatesfrom a first position on a first side of the first slab coupler to asecond position on a second side of the second slab coupler, acorrespondence between said first and second positions depending upon awavelength of the optical signal; a plurality of array waveguidescoupled between a second side of the first slab coupler and a first sideof the second slab coupler; and a plurality of output waveguides coupledat one end to the second side of the second slab coupler; wherein theoutput waveguides are arranged to fan-out from the second slab couplerin a fan-out region of the device which comprises an initial portion ofthe length of each output waveguide, and the waveguides in the fan-outregion are substantially identically curved in at least portion of thefan-out region adjacent the respective tapered ends thereof
 2. An AWGdevice according to claim 1, wherein the output waveguides all havesubstantially the same radius of curvature in at least a portion of thefan-out region.
 3. An AWG device according to claim 1, wherein theoutput waveguides each have a continuously varying radius of curvature,in the fan-out region, and the radius of curvature of each waveguidevaries substantially identically to that of the other waveguides, alongcorresponding portions of the lengths thereof, in at least a portion ofthe fan-out region.
 4. An AWG device according to any preceding claim,wherein the output waveguides are substantially single mode waveguideswhich are tapered in width at the ends thereof which are coupled to thesecond slab coupler, so as to increase in width towards the second slabcoupler.
 5. An AWG device according to claim 1, wherein the radius ofcurvature of said substantially identically curved portions is keptsufficiently low to substantially filter out any pseudo higher ordermodes which arise where the output waveguides are coupled to the secondslab coupler, while still being sufficiently high to avoid radiationlosses above a predetermined maximum level, in the output waveguides. 6.An AWG device according to claim 5, wherein the output waveguides eachcomprise a core region having cladding material at least on either sideof the core region, the difference between the refractive indices of thecore region and the cladding material is 0.01, and the size of the coreregion is 6×6 μm in cross-section, and wherein the radius of curvatureof the waveguides in the portion of the fan-out region in which thewaveguides are substantially identically curved is in the range of 5.5to 16 mm.
 7. An AWG device according to claim 6, wherein the radius ofcurvature of the output waveguides in the portion of the fan-out regionin which the waveguides are substantially identically curved issubstantially 8 mm.
 8. An AWG device according to any preceding claim,wherein the output waveguides are substantially identically curvedthroughout a substantial portion of the fan-out region.
 9. An AWG deviceaccording to claim 8, wherein the device is of generally planarrectangular shape, having an input edge, an output edge and two sideedges, a second end of each output waveguide lies along the output edgeof the device, and the portion of the fan-out region in which the outputwaveguides are substantially identically curved has a length in therange of 3-20% of the length of the fan-out region, as measured along asaid side edge of the device which is perpendicular to the output edgeof the device.
 10. An AWG device according to claim 9, wherein theportion of the fan-out region in which the output waveguides aresubstantially identically curved has a length in the range of 5-10% ofthe length of the fan-out region, as measured along said side edge ofthe device which is perpendicular to the output edge of the device. 11.An AWG device according to any preceding claim, wherein the waveguidesare identically curved in at least a portion of the fan-out region. 12.An AWG device according to any one of claims 1 to 10, wherein each ofsaid substantially identically curved portions of the waveguides differsin curvature by no more than 1 mm radius of curvature from any othersaid substantially identically curved waveguide portion.
 13. An AWGdevice according to claim 12, wherein each of said substantiallyidentically curved portions of the waveguides differs in curvature by nomore than 0.5 mm radius of curvature from any other said substantiallyidentically curved waveguide portion.
 14. An AWG device according to anypreceding claim, further including a plurality of input waveguidescoupled to a first side of the first slab coupler each input waveguideis tapered in width at the end thereof which is coupled to the firstslab coupler, so as to increase in width towards the first slab couplerand the input waveguides are arranged to fan-in towards the first slabcoupler, in a fan-in region which comprises a portion of the length ofeach input waveguide, and wherein all the input waveguides aresubstantially identically curved in at least a portion of the fan-inregion.
 15. An AWG device according to claim 14, wherein thesubstantially curved portions of the input waveguides are substantiallyinversely curved to said substantially identically curved portions ofthe output waveguides, with respect to the direction of travel of lightalong the waveguides.
 16. An AWG according to claim 15, wherein thesubstantially identically curved portions of the input waveguides have aradius of curvature which is of equal magnitude to said substantiallyidentically curved portions of the output waveguides.
 17. An AWGaccording to claim 15 or 16, wherein the output waveguides are taperedin width at the ends thereof where they are coupled to the second slabcoupler, said substantially identically cursed portions of the inputwaveguides are adjacent the respective tapered ends of the inputwaveguides, and the substantially identically curved portions of theoutput waveguides are adjacent the respective tapered ends thereof. 18.An arrayed waveguide grating comprising: a substrate having first andsecond slab couplers; a plurality of array waveguides optically coupledbetween the first and second slab couplers and having respectivepredetermined optical path length differences therebetween; at least oneinput waveguide being tapered in width at a first end and beingoptically coupled at said first end to an input side of the first slabcoupler; a plurality of output waveguides each being tapered in width ata first end thereof and being optically coupled at said first end to anoutput side of the second slab coupler; the output waveguides beingarranged to fan-out from the second slab coupler in a fan-out region ofthe device which comprises an initial portion of the length of eachoutput waveguide; wherein the output waveguides in the fan-out regionare substantially identically curved in a portion adjacent therespective tapered ends thereof; the or each said input waveguide issubstantially inversely curved to said substantially identically curvedportion of the output waveguides, with respect to the direction oftravel of light along the waveguides, along a corresponding portion ofsaid input waveguide adjacent the respective tapered ends thereof. 19.An AWG according to claim 18, wherein the or each said input waveguidehas equal magnitude radius of curvature to said substantiallyidentically curved portions of the output waveguides.
 20. Amultiplexer/demultiplexer comprising an AWG device according to anypreceding claim.
 21. A communications system incorporating at least oneAWG device according to any of claims 1 to
 19. 22. An AWG devicesubstantially as described herein with reference to FIG. 3(b).
 23. AnAWG device substantially as described herein with reference to FIGS.7(a) and (b).